Many different kinds of concealed weapons detection systems are known and are used in a wide range of situations in order to provide added security against violent crimes and terrorist attacks. For example, in addition to the well-known uses of concealed weapons detection systems in airports, weapons detection systems are increasingly being used in court houses, schools, and other public and governmental facilities that may be subject to threats or attacks.
One type of concealed weapons detection system utilizes a plurality of magnetic sensors positioned adjacent a sensing area or portal through which pass persons and/or objects (hereinafter referred to as “subjects”) to be “scanned” by the weapons detection system. The magnetic sensors of the weapons detection system detect changes or disturbances in an ambient magnetic field (e.g., the earth's magnetic field) caused by a ferromagnetic object (e.g., a weapon) in the sensing area or portal. If a ferromagnetic object of sufficient size is detected, the weapons sensor activates an alarm.
While such magnetic detectors are capable of detecting ferromagnetic objects passing through the detector portal, problems remain in determining whether the detected ferromagnetic object is a threat item (e.g., a gun or a knife) or a non-threat item (e.g., a steel shank of a shoe or a belt buckle). Incorrectly classifying a non-threat item as a threat item results in a false alarm. Of course, incorrectly classifying a threat item as a non-threat item is unacceptable as well.
One type of weapons detection system seeks to distinguish between threat items and non-threat items by analyzing the magnetic “signature” produced by the ferromagnetic object as it passes through the portal. Unfortunately, however, it has proven difficult to reliably distinguish between the magnetic signatures produced by threat items and non-threat items. One difficulty stems from the fact that the magnetic sensors used by such weapons detection systems are highly sensitive to magnetic fields. As a result, the output signals of the sensors often contain magnetic signatures from other sources, such as relatively small ferromagnetic objects (e.g., keys or pocket change) on the subject or relatively large ferromagnetic objects located nearby, but outside, the detector portal. In addition, the detectors may also detect stray magnetic fields produced by various types of electrical equipment.
Other factors affecting the ability to successfully distinguish between threat items and non-threat items relate to the relative position of the ferromagnetic object with respect to the magnetic sensors of the portal. Variations in the speed of passage of the object through the portal can also affect the ability to distinguish between threat items and non-threat items. In addition, the vertical position of the ferromagnetic object with respect to the portal, the magnetic signatures resulting from other ferromagnetic objects that may be on the subject, as well as the particular gait through the portal of a person carrying such ferromagnetic objects, all add to the difficulty in distinguishing threat items from non-threat items.
As a result of these problems, a need remains for a weapons detection system that can more accurately and reliably distinguish between a threat item, such as a gun or a knife, and a non-threat item, such as a belt buckle or a steel shank in a shoe.